58 The Bad land Formations of the Black Hill® Region 



tensive. The Arikaree he considers as a flat alluvial fan of 

 wonderful extent probably spread out over the Plains region by 

 streams aided to a minor extent by winds. 



Prof. Osborn has quite recently summed up all of the evid- 

 ence and states that the present opinion appears to be as follows : 



"The topography of the Plains Region was in Oligocene 

 to lower Pleistocene time, as now, level or gently undulating, 

 not mountainous. On the gentle eastward slopes of the Rocky 

 Mountains and the Black Hills were borne broad streams with 

 varying channels, backwaters, and lagoons, sometimes spread- 

 ing into shallow lakes, but never into vast fresh-water sheets. 

 Savannas were interspersed by grass-covered pampas, traversed 

 by broad, meandering rivers which frequently changed their 

 channels. This accounts for the presence of true conglomerates, 

 true sandstones, calcareous grits, gypsum, fine clays, fullers' 

 earth, fine loess, eolian sands, and even, far out on the plains of 

 Nebraska and Kansas (and South Dakota) widespread deposits 

 of volcanic dust, wind borne from distant craters in the moun- 

 tains to the west and southwest. In the early Oligocene and 

 Miocene the deposits were chiefly fluviatile or river sandstones 

 and conglomerates interspersed with fine flood plain or over- 

 flow deposits, perhaps locally lacustrine, partly of volcanic 

 ashes. As the dessication or aridity of the country increased, 

 the mountain-fed rivers became smaller and narrower, while 

 the eolian or loess deposits apparently became more common, 

 beginning in the middle Miocene. The deposits also became 

 more and more restricted in extent as the Miocene advanced. 

 The newer river channels cut down into the older series, thus 

 using the erosion materials a second time."* 



From the above it may be seen that in large measure the- 

 determination of this question of manner of deposition, like so 

 many other nature puzzles, seems to have waited merely more 

 refined investigation. This has now been supplied with some 

 degree of fulness by the various field parties, and while there 

 are yet doubts as to the relative importance of certain features 

 and much work still to be done, Prof. Osborn's summary so far 

 as present knowledge is concerned may be taken as adequate 

 and satisfactory. 



SOURCE OF MATERIALS 



The immediate source of 'the badland deposits is not 

 definitely known. The material was evidently derived from 



*U. S. Geol. Surv., Bull. 361, p. 28. 



